Configurable out-of-order data transfer in a coprocessor interface

ABSTRACT

An interface for transferring data between a central processing unit (CPU) and a plurality of coprocessors is provided. The interface includes an instruction bus and a data bus. The instruction bus is configured to transfer instructions to the plurality of coprocessors in an instruction transfer order, where particular instructions designate and direct one of the plurality of coprocessors to transfer the data to/from the CPU. The data bus is configured to subsequently transfer the data. Data order signals within the data bus prescribe a data transfer order that differs from the instruction transfer order by prescribing a transfer corresponding to a specific outstanding particular instruction, where the data transfer order is relative to outstanding instructions. The outstanding instructions are those of the particular instructions transferred to the one of the plurality of coprocessors that have not completed a data transfer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No. 09/753,239 entitled “COPROCESSOR INTERFACE TRANSFERRING MULTIPLE INSTRUCTIONS SIMULTANEOUSLY ALONG WITH ISSUE PATH DESIGNATION AND/OR ISSUE ORDER DESIGNATION FOR THE INSTRUTIONS”; U.S. patent application Ser. No. 09/751,748 entitled “HIGHLY CONFIGURABLE CO-PROCESSOR INTERFACE”; and U.S. patent application Ser. No. 09/751,746 entitled “A COPROCESSOR INTERFACE ENABLING COPROCESSOR-SPECIFIC BRANCHING”; each of which are incorporated herein by reference for all purposes.

FIELD OF THE INVENTION

This invention relates in general to the field of computer architecture, and more specifically to an interface for performing out-of-order data transfers between a central processor and one or more coprocessing devices.

BACKGROUND OF THE INVENTION

Early microprocessor based systems utilized a central processing unit, or CPU, which executed instructions, one at a time, in the order in which they were presented. These CPU's, due to their relative complexity, and associated cost, were typically designed to have universal application. That is, they were designed as general purpose CPU's.

As the use of general purpose CPU's increased, so did the variety of application programs they were required to execute. In some instances, certain programs ran extremely slow because they required the CPU to perform complex calculations that were really beyond the scope of what it was intended to perform. An example of such calculations included floating point operations, such a multiply, divide, etc.

To assist the CPU in executing such complex calculations, a floating point coprocessor was designed. The floating point coprocessor was essentially a second processor, external to the CPU, that was designed to perform certain complex operations such as a floating point multiply, a floating point divide, etc, albeit much faster than the CPU. In operation, the CPU, when it was asked to perform a complex operation, would pass the operation over to the floating point coprocessor. The coprocessor would complete the task, and pass the results back to the CPU.

Although the development of the floating point coprocessor enhanced the processing speed of certain applications, its cost kept it from being added to all systems. So, many computer systems were designed around a particular CPU (e.g., an 80286), and were designed to accommodate a dedicated floating point coprocessor (e.g., an 80287), but the coprocessor was not provided with the system. If a user of the system executed the types of applications that could take advantage of the coprocessor, for an additional cost s/he could add the coprocessor to the system.

Thus, while development of the coprocessor enhanced the performance of many CPU based systems, certain design problems arose. The first is that the coprocessor and the general purpose CPU must be tightly integrated, or designed together, to allow them to communicate together effectively. Operationally speaking, the CPU must be able to detect when a program wishes to use the coprocessor, it must pass those instructions over to the coprocessor, it must provide the data to the coprocessor that corresponds to the instructions, and it must be able to receive results back from the coprocessor. In addition, the coprocessor must know whether the CPU has been interrupted by another process, and whether it still requires the result that it is calculating. One skilled in the art will appreciate that many other issues arise relating to communications that must occur between a CPU and a coprocessor. Consequently, since the CPU and the coprocessor have to be so tightly integrated, most coprocessors that have been designed are typically designed to work with a particular CPU.

A problem that stems from the tight integration that is required between a CPU and coprocessor is that enhancement of the design of a particular CPU often requires a corresponding enhancement (or change) in the design of its associated coprocessor. For instance, an older coprocessor will typically not function with a newer CPU, and a newer coprocessor will typically not function with an older CPU. Thus, every time a CPU manufacturer wishes to introduce a new CPU, they must decide whether they want to develop a dedicated coprocessor that will work along with it. A decision to develop a coprocessor is a very costly decision that ultimately must be supported by the marketplace.

In addition, although the above-described history of the floating-point coprocessor provides a basis for understanding that the coprocessor must be designed with a specific CPU in mind, it does not address the scope of today's coprocessing problems. Certain present day application programs require complex, time-consuming calculations that are neither appropriate for a CPU, nor for a floating-point coprocessor; i.e., other types of coprocessor's are required to optimally execute these application programs. Such programs perform 3-D rendering for graphics applications, audio/signal processing etc. But graphics coprocessors, audio coprocessors, etc., that are required to perform these special-purpose programs typically must provide a particular interface (e.g., AGP) and thus these coprocessors will only inter-operate with CPU's that are designed for that particular interface as well.

From the viewpoint of a system designer, choosing a particular CPU for a particular application is increasingly difficult. The designer must anticipate the future needs of the system, utilizing tightly coupled CPU/coprocessor products, without being able to select the CPU and its coprocessors separate from one another. For example, a designer may have requirements within an application that may be satisfied by choosing a relatively simple CPU along with a fairly complex graphics coprocessor. Regrettably, the complex coprocessor today is only compatible with an equally complex (and costly) CPU. Alternatively, the designer may have requirements within an application that may be satisfied by selecting a complex CPU along with a simple coprocessor. This combination is unfortunately not possible either because the complex CPU only interfaces to an equally complex (and costly) coprocessor.

Therefore, what is needed is a configurable coprocessor interface that allows a variety of CPU's to be easily coupled to a variety of different coprocessors, without requiring that the coprocessors be designed specifically for those CPU's.

What is also needed is a configurable coprocessor interface that allows both forward and backward compatibility between CPU's and coprocessors. That is, a coprocessor interface is desired that allows older, or legacy, coprocessors to be utilized with newer CPU's, and vice versa.

Further, what is needed is a “standardized” coprocessor interface for which CPU's and coprocessors can be designed. Such an interface would allow a system designer to select those specific CPU and coprocessor combinations that provide an optimum solution for the designer's needs, without regard to proprietary interface requirements.

Finally, what is needed is a coprocessor interface that takes advantage of modern processing technology, such as multiple-issue of instructions, out-of-order data transfer, etc., without mandating that such technology exist in every device on the interface.

SUMMARY OF THE INVENTION

The present invention provides a scalable and configurable coprocessor interface that distinguishes between instruction types that are transferred between a central processing unit (CPU) and a coprocessor. The configurable coprocessor interface also allows sequential or parallel transfer of differing instruction types to one or more coprocessor pipelines. In addition, the interface provides separate TO/FROM data buses between the CPU and the coprocessor to allow for simultaneous data transfer (in/out) between the CPU and the coprocessor. Furthermore, the interface provides for disassociation between instructions that are transferred, and data that is transferred, to allow data elements to be moved in variable time slots with respect to their associated instructions. Moreover, the interface allows for out-of-order data elements to be transferred between the CPU and its coprocessors in an order that is not tied to the order that associated instructions are transferred (i.e., out-of-order data transfer). Out-of-order data transfer according to the present invention does not require order tags to be associated with each data transfer. Rather, the interface keeps track of the relative order of outstanding instructions that require data for execution, and provides a relative order indicator along with each piece of data as it is transferred. In addition, condition code signaling is provided from the coprocessor to allow the coprocessor to evaluate CPU specific conditional instructions, and to inform the CPU as to whether or not it should execute the CPU conditional instructions.

An embodiment of the present invention provides an interface for transferring data between a central processing unit (CPU) and a plurality of coprocessors. The interface includes an instruction bus and a data bus. The instruction bus transfers instructions to the plurality of coprocessors in an instruction transfer order, where particular instructions designate and direct one of the plurality of coprocessors to transfer the data to/from the CPU. The data bus is coupled to the instruction bus. The data bus subsequently transfers the data, where data order signals within the data bus prescribe a data transfer order that differs from the instruction transfer order, and where the data order signals prescribe transfer of a data element. The data element corresponds to a specific outstanding instruction, where the data order is relative to outstanding instructions, the outstanding instructions being those of the particular instructions transferred to the one of the plurality of coprocessors that have not completed a data transfer. The interface keeps track of said data order, and where said data order signals indicate said data order, and where said data order signals are provided with said data element as said data element is transferred.

In another aspect, the present invention provides a computer program product for use with a computing device, the computer program product has a computer usable medium that includes computer readable program code embodied thereon. The computer readable program code, when used with the computing device, causes enablement of functions corresponding to a coprocessor interface that transfers data between CPU and a plurality of coprocessors. The computer readable program code has first program code and second program code. The first program code causes enablement of functions corresponding to an instruction bus, where the instruction bus is configured to transfer instructions to the plurality of coprocessors in an instruction transfer order, and where particular instructions direct designated ones of the plurality of coprocessors to transfer the data to/from the CPU. The second program code causes enablement of functions corresponding to a data bus. The data bus is configured to subsequently transfer the data, where data order signals within the data bus prescribe a data transfer order that is different from the instruction transfer order, and where the data order signals prescribe transfer of a data element. The data element corresponds to a specific outstanding instruction. The data order is relative to outstanding instructions. The outstanding instructions are those of the particular instructions transferred to the one of said plurality of coprocessors that have not completed a data transfer. The coprocessor interface keeps track of the data order, where the data order signals indicate the data order, and where the data order signals are provided with the data element as the data element is transferred.

In a further aspect, the present invention provides a an apparatus for transmitting program code. The apparatus includes a communication network having a transmission medium, for embodying a computer data signal therein. The computer data signal has computer-readable first program code and computer-readable second program code. The computer-readable first program code is for use with a computing device, where the computer-readable first program code, when used with the computing device, causes enablement of functions corresponding to an instruction bus for transferring instructions to a plurality of coprocessors in an instruction transfer order, where particular instructions direct particular coprocessors to transfer data to/from a CPU. The computer-readable second program code is for use with the computing device, where the computer-readable second program code, when used with the computing device, causes enablement of functions corresponding to a data bus for subsequently transferring the data, where data order signals within the data bus prescribe a data transfer order that differs from the instruction transfer order, and where the data order signals prescribe transfer of a data element. The data element corresponds to a specific outstanding instruction. The data transfer order is relative to outstanding instructions. The outstanding instructions are those of the particular instructions transferred to the particular coprocessor that have not completed a data transfer.

In yet another aspect, the present invention provides a method for transferring data between a CPU and a plurality of coprocessors. The method includes transmitting instructions to the plurality coprocessors, each of the instructions designating the one of the plurality of coprocessors, and each of the instructions directing a data transfer between the CPU and a specific coprocessor, where the transmitting is provided in a specific instruction order; and subsequently transferring the data in an order different from the specific instruction order. The transferring includes specifying transfer of a data element corresponding to a specific outstanding instruction, where the order of said transferring is relative to outstanding instructions, the outstanding instructions being those of the instructions transmitted to the one of the plurality of coprocessors that have not completed a data transfer; and providing the data order signals with the data element as the data element is transferred.

Other features and advantages of the present invention will become apparent upon study of the remaining portions of the specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a central processing unit (CPU) coupled to a coprocessor utilizing the coprocessor interface of the present invention.

FIG. 2 is a flow chart illustrating instruction flow through the CPU and coprocessor of FIG. 1.

FIG. 3 is a block diagram illustrating instruction dispatch signals between a CPU and a coprocessor utilizing the coprocessor interface of the present invention.

FIG. 4 is a timing diagram illustrating how instructions are dispatched on the coprocessor interface of the present invention.

FIG. 5 is a timing diagram illustrating how multiple instructions are dispatched to multiple issue groups via the coprocessor interface of the present invention.

FIG. 6 is a timing diagram illustrating how the coprocessor delays instruction dispatch from the CPU utilizing busy signals on the coprocessor interface of the present invention.

FIG. 7 is a block diagram illustrating coprocessor to/from data transfer instruction signals between a CPU and a coprocessor utilizing the coprocessor interface of the present invention.

FIG. 8 is a timing diagram illustrating how data is transferred from a CPU to a coprocessor utilizing the coprocessor interface of the present invention.

FIG. 9 is a timing diagram illustrating how data is transferred from a coprocessor to a CPU utilizing the coprocessor interface of the present invention.

FIG. 10 is a block diagram illustrating coprocessor interface signals for: 1) coprocessor condition code checks; 2) CPU general purpose register values; 3) exception signal from a coprocessor to the CPU; and 4) instructions commit signaling from a CPU to a coprocessor, according to the present invention.

FIG. 11 is a timing diagram illustrating how a coprocessor responds to a CPU with condition code checks, according to the present invention.

FIG. 12 is a timing diagram illustrating exception handling between a CPU and a coprocessor utilizing the coprocessor interface of the present invention.

FIG. 13 is a timing diagram illustrating an instruction commit procedure by a CPU coupled to a coprocessor utilizing the coprocessor interface of the present invention.

DETAILED DESCRIPTION

Referring to FIG. 1, a portion of a computing system 100 is shown. The computing system 100 provides a central processing unit (CPU) 102 coupled to memory 120 via a system bus 118. In general, the CPU 102 executes instructions retrieved from the memory 120, and interfaces with other devices 124 that are external to it via an I/O interface 122 attached to the system bus 118.

The CPU 102 is also coupled to a coprocessor 110 utilizing a coprocessor interface according to the present invention. The coprocessor 110 may be any type of coprocessor, but most commonly is a floating-point coprocessor, or a graphics (3-D) coprocessor. As will be more fully described below, with reference to FIG. 2, the CPU 102 retrieves instructions from the memory 120, and determines whether the instructions are directed to it, or to the coprocessor 110. If the instructions are to be directed to the coprocessor 110, they are provided using the coprocessor interface of the present invention.

Within the CPU 102 are a bus interface 104, a cache system 106, an execution core 108, and a coprocessor interface 112 a. The bus interface 104 is coupled to the system bus 118 for retrieving and storing instructions and data, from and to the memory 120, and for communicating with the other devices 124. Data and instructions retrieved by the bus interface are provided to the cache system so that they may be immediately accessible to the execution core 108. The execution core 108 executes instructions provided to it by the cache system 106, operating on data that is either resident within the cache system 106, or retrieved by the bus interface 104.

In one embodiment, the execution core 108 is split into two (or more) execution units (designated A & B), each capable of executing instructions directed to the CPU 102. That is, two (or more) instructions may be provided to the execution units A & B, within the execution core 108, for simultaneous execution.

The execution core 108 is coupled to a coprocessor interface (COP interface) 112 a. The COP interface 112 a enables coprocessors such as floating point units (FPU's) and Graphics Engines to be tightly coupled to an integer processor core such as the execution core 108. In one embodiment, the COP interface 112 a is designed to allow coprocessors to interface with various processor cores designed by MIPS Technologies, of Mountain View, Calif.

The COP interface 112 a includes many features, each of which will be further described below, with reference to FIGS. 3–14. More specifically, the COP interface 112 a allows 32-bit or 64-bit data transfer between an integer core and a coprocessor. In addition, the COP interface 112 a supports one or more coprocessors, each of which may have one or more execution units. The COP interface 112 a allows between 0 and 8 out-of-order data transfers. And, the COP interface 112 allows either single-issue of instructions, or multiple instruction issue, to one or more coprocessors, where each issue supports arithmetic and/or data transfers. Furthermore, as will be more particularly described with reference to FIGS. 10, 11, and 13, the COP interface 112 allows for user defined exceptions, and coprocessor defined condition code interpretation. Finally, the COP interface 112 a is designed to support coprocessor instructions defined in the MIPS32, MIPS64, MDMX and MIPS-3D architecture specifications available from MIPS Technologies, Inc. Of course, one skilled in the art will appreciate that the novel features of the COP interface 112 a are not restricted to data size, instruction set architecture, etc. Rather, the COP interface 112 a is applicable to any interface between a CPU and a coprocessor where the design considerations include scalability, compatibility (both forward and backward), and/or configurability.

As shown in FIG. 1, the COP interface 112 a within the CPU 102 is coupled to a coprocessor 110 via coprocessor interface signal lines 116. Within the coprocessor 110 are a COP interface 112 b and an execution unit 114. The COP interface 112 b is similar to the COP interface 112 a, but is designed to communicate from the coprocessor 110 to the CPU 102. The execution unit 114 may either be a single-issue execution unit, or may be split into two (or more) execution units A and B for simultaneous execution of multiple coprocessor instructions.

Operation of the computing system 100 will now be described with respect to the flow of instruction execution by reference to the flow chart 200 of FIG. 2, to which attention is now directed.

Instruction flow begins when instructions are fetched 202 by the execution core 108. That is, a program counter (not shown) contains an address related to the next instruction to be executed. This address is provided to the cache system 106 for determination of whether the instruction is resident within the cache system 106. If it is not, the instruction is first fetched from the memory 120, by the bus interface 104 and is provided to the execution core 108. Once the instruction is provided to the execution core 108, the instruction flow proceeds to the decode block 204.

At decode block 204, the execution core decodes the instruction to be executed. That is, the decode block performs a variety of decoding functions, including determining whether the instruction is a branch instruction, and if so, predicting whether the branch will be taken, determining what operands are necessary for execution of the instruction, etc. Instruction flow then proceeds to decision block 206.

At decision block 206, the execution core 108 determines whether the instruction to be executed is an integer instruction to be executed by the execution core 108, or whether the instruction is a coprocessor instruction to be executed by the execution unit 114. If the instruction is an integer instruction, instruction flow proceeds to block 208. Otherwise, the execution core 108 provides the coprocessor instruction to the COP interface 112 a. The COP interface 112 a then presents the coprocessor instruction, and any associated data for the instruction, to the coprocessor 110 for execution. Instruction flow then proceeds to block 212.

At block 208, the execution core 108 executes the integer instruction. Instruction flow then proceeds to block 210 where the result of the execution, if any, is written back into a register (or memory location) within the CPU 102.

At block 212, the COP interface 112 b receives the coprocessor instruction from the CPU 102 for execution. The coprocessor instruction, and its associated data, are provided to the execution unit 114 for execution. The result of the execution remains in the coprocessor 110 until future instructions request that the result be transferred to the CPU 110.

The above discussion, with reference to FIGS. 1 and 2 provides a general understanding of an interface between a CPU 102 and a coprocessor 110, utilizing the coprocessor interface 112 of the present invention. With the above in mind, particular operational specifics of the COP interface 112 of FIG. 1 will now be described with reference to FIGS. 3–14 by description of specific interface signals within the COP interface 112, and their operation with respect to such functions as instruction transfers, data TO transfers, data FROM transfers, etc.

Instruction Transfer

Instruction transfer refers to how coprocessor instructions are provided from a CPU to a coprocessor. Table 1 below provides a brief summary of some of the instruction transfer signals that are provided within the COP interface of the present invention. Illustration of how these signals operate will be further described below with reference to FIGS. 3–6.

TABLE 1 Issue Signal Name Dir Group Description CP_ir_m[31:0] Out Comb, Coprocessor Instruction Word. Valid in the cycle before Arith, CPx_as_m, CPx_ts_m, CPx_fs_m is asserted. TF CPx_as_m Out Comb, CoprocessorX Arithmetic Instruction Strobe. Asserted Arith in the cycle after an arithmetic coprocessorX instruction is available on CP_ir_m. If CPx_abusy_m was asserted in the previous cycle, this signal will not be asserted. In any cycle, at most 1 of the following signals can be asserted at a time in a particular issue group. CPx_as_m, CPx_ts_m, CPx_fs_m. CPx_abusy_m In Comb, CoprocessorX Arithmetic Busy. When asserted, a Arith coprocessorX arithmetic instruction will not be dispatched. CPx_as_m will not be asserted in a cycle after the signal is asserted. CPx_ts_m Out Comb, CoprocessorX to Str obe. Asserted in the cycle after a TF To COPx Op instruction is available on CP_ir_m. If CPx_tbusy_m was asserted in the previous cycle, this signal will not be asserted. In any cycle, at most 1 of the following signals can be asserted at a time in a particular issue group: CPx_as_m, CPx_ts_m, CPx_fs_m. CPx_tbusy_m In Comb, To CoprocessorX Busy. When asserted, a To COPx Op Arith instruction will not be dispatched. CPx_ts_m will not be asserted in a cycle after the signal is asserted. CPx_fs_m Out Comb, CoprocessorX From Strobe. Asserted in the cycle after a TF From COPx Op instruction is available on CP_ir_m. If CPx_fbusy_m was asserted in the previous cycle, this signal will not be asserted. In any cycle, at most 1 of the following signals can be asserted at a time in a particular issue group: CPx_as_m, CPx_ts_m, CPx_fs_m. CPx_fbusy_m In Comb, From CoprocessorX Busy. When asserted, a From Arith COPx Op instruction will not be dispatched. CPx_fs_m will not be asserted in a cycle after the signal is asserted. CP_order_m[2:0] Out Comb, Coprocessor Dispatch Order. Signifies the program Arith, order of instructions when more than one instruction is TF issued in a single cycle. Each instruction dispatched has an order value associated with it. There must always be one instruction whose order value is 0. Order values must increment by 1 when more than one instruction is issued in a cycle. Valid when CPx_as_m, CPx_ts_m, or CPx_fs_m is asserted. CP_VNs_m Out Comb, Coprocessor Valid or Null Strobe. Asserted when a valid Arith, or null signal is available on CP_VN_m. TF CP_VN_m Out Comb, Valid or Nullify Coprocessor Instruction. When Arith, deasserted, the integer processor core is signaling that the TF instruction is valid, i.e., not nullified. When asserted, the integer processor core is signaling that the instruction is nullified. Valid when CP_VNs_m is asserted.

Signal names in Table 1, and in the other tables that follow should be interpreted as follows:

Signal names beginning in CP_relate to signals from a CPU to any/all coprocessors coupled to the CPU. Signal names beginning in “CPx”_ are specific to particular coprocessors attached to the CPU. That is, if two coprocessors are attached to a CPU, two sets of signal lines will be provided by the coprocessor interface within the CPU. The first set of signal lines will begin “CP1_”, the second set of signal lines will begin “CP2_”, etc. Rather than illustrate separate signals for CP1 and CP2, they have been combined in the present discussion as “CPx” for purposes of clarity. Note: in one embodiment, within the context of the MIPS architecture, the only valid coprocessors are CP1 and CP2.

In addition, instruction types have been designated as arithmetic (a), data TO coprocessor (t), or data FROM coprocessor (f). So, arithmetic signal names include the letter “a”, TO signal names include the letter “t”, and FROM signal names include the letter “f”. By distinguishing between arithmetic, data to, and data from instructions, the coprocessor interface of the present invention allows coprocessors to disable certain types of instructions, within particular issue groups.

The ending designation “_m” designates a particular issue group. The coprocessor interface of the present invention is extensible to support both single-issue and multi-issue cores and coprocessors. That is, a single issue CPU can support one or more multi-issue coprocessors. Alternatively, a multi-issue CPU can utilize single or multi-issue coprocessors. Multi-issue support is achieved by duplicating certain signals of the coprocessor interface. Thus, if support of single-issue is desired, only one set of signals is necessary, with the signal names ending in _m represented by “_0”. If multi-issue support is desired, then two sets of signals are necessary, with signal names ending in m represented by “_0” for the first set, and “_1” for the second set, etc.

In one embodiment, all transfers between a CPU and a coprocessor are synchronously strobed. That is, a transfer, whether instruction or data, is only valid for one cycle (when a strobe signal is asserted). In addition, there is no handshake confirmation of transfer, nor is there flow control relating to the transfer. Furthermore, except for data transfers, out-of-order transfers are not allowed. That is, all transfers of a given type, within the same issue group (to be further described below), must be in instruction dispatch-order. However, ordering of different types of transfers for the same instruction is not restricted.

Referring now to FIG. 3, a block diagram 300 is shown illustrating particular instruction dispatch signals 316 between a CPU 302 and a coprocessor 310 that were summarized above in Table 1. Each of the CPU 302 and the coprocessor 310 contains a COP interface 312 according to the present invention to allow instruction dispatch as follows.

FIG. 4 provides a timing diagram 400 illustrating a simple case of instruction transfer. That is, three coprocessor instructions A, B & C will be transferred by the CPU 302 to the coprocessor 310. All of the signals shown in FIG. 4 are synchronous with respect to a common clock signal generated by the CPU 302. In one embodiment, the clock signal is the core frequency signal of the CPU 302.

At clock cycle 2, a coprocessor instruction A has been placed by the coprocessor interface 312 within the CPU 302 on CP_ir_m[31:0]. This instruction is an arithmetic instruction, such as an ADD, MULTIPLY, DIVIDE, but may be another type of instruction defined within the arithmetic group. In addition, this instruction is in the issue group designated by “_m”. At the beginning of clock cycle 2, it is said that instruction A has been dispatched by the coprocessor interface 312.

After an instruction is dispatched, additional information about that instruction must be transferred between the CPU 302 and the coprocessor 310. Such information includes the type of instruction (a, t, or f) an indication of which coprocessor the instruction is destined for (CP1, CP2, etc.), and the issue group to which the instruction belongs (0, 1, 2 . . . ).

At clock cycle 3, the coprocessor interface 312 asserts the instruction dispatch strobe signal CPx_as_m to indicate to the coprocessor 310 that the instruction dispatched during the previous cycle should be used by the coprocessor as an active instruction, and that it was an arithmetic instruction. It is possible that the instruction dispatch strobe signal may be asserted in the same cycle as the instruction dispatch.

Each such transfer can occur as early as one cycle after instruction dispatch, with no maximum limit on how late the transfer can occur. That is, only the dispatch portion of the coprocessor interfaces have flow control. In addition, the coprocessor interface 312 is designed to operate with coprocessors of any pipeline structure and latency. If the CPU 302 requires a specific transfer by a certain cycle, the CPU 302 must stall until the transfer has completed. In addition, all transfers are strobed. That is, independent of strobing, data is transferred in the cycle that the strobe signal is asserted. If the strobe signal is asserted for 2 cycles, then two transfers occur.

At clock cycle 4, an instruction B is dispatched on signal lines CP_ir_m.

At clock cycle 5, an instruction C is dispatched on signal lines CP_ir_m. In addition, the coprocessor interface 312 asserts a CPx_ts_m signal to the coprocessor 310 to indicate that the instruction dispatched on the previous cycle (instruction B) should be used as a TO coprocessor data instruction.

At clock cycle 6, the coprocessor interface 312 asserts a CPx_ts_m signal to the coprocessor 310 to indicate that the instruction dispatched in the previous cycle (instruction C) should be used as a TO coprocessor data instruction.

At this point, three instructions have been transferred by the coprocessor interface 312 to the coprocessor 310: an arithmetic instruction A, and two TO coprocessor data instructions B and C. One skilled in the art should appreciate, however, that the instructions could have been dispatched to different coprocessors, if available, by utilizing separate strobe lines designated by “CP1_ts_m” and “CP2_ts_m”, for example.

Instruction Valid or Nullify

All instructions that are dispatched must be indicated as valid or nullified by the execution core after dispatch to the coprocessor 310. The valid or nullify transfer signals within the coprocessor interface 312 must occur so that the coprocessor 310 knows when it can begin operation of subsequent operations that depend on the result of the current instruction. Validity or Nullification must be performed in an early stage of the coprocessor 310's pipeline to insure that subsequent instructions can begin with correct operands.

Referring to FIG. 4, on clock cycle 4, the coprocessor interface 312 asserts strobe signal CP_VNs_m. This assertion corresponds to a validity/nullification transfer for instruction A. Also during clock cycle 4, the coprocessor interface 312 deasserts signal CP_VN_m to indicate that instruction A is valid and should not be nulled.

On clock cycle 5, the coprocessor interface 312 continues to assert strobe signal CP_VNs_m, and continues to deassert signal CP_VN_m to indicate that instruction B is valid.

On clock cycle 8, the coprocessor interface 112 asserts strobe signal CP_VNs_m corresponding to a validity or nullification transfer for instruction C. Also during clock cycle 8, the coprocessor interface 312 asserts signal CP_VN_m to indicate to the coprocessor that instruction C should be nulled.

Thus, instruction validity or nullification continues for all instructions transferred between the CPU 302 and the coprocessor 310. There is no tag or other identification relating a validity or nullification signal on CP_VN_m with an instruction. Rather, since the instructions are in-order, the coprocessor interfaces within both the CPU 302 and the coprocessor 310 keep track of each transferred instruction, and do not allow an instruction to use operands generated by previous instructions until those previous instructions have received a CP_VN_m signal. In addition, if an instruction is nullified, no remaining transfers for that instruction will occur. If a transfer related to a nullified instruction occurs during the cycle of nullification, the transfer will be ignored.

Instruction Ordering—Multiple Issue Groups

Referring now to FIG. 5, a timing diagram 500 is provided to particularly illustrate how instructions are transferred to multiple issue groups. In this timing diagram, it is presumed that the CPU 302 is a dual issue integer processor. The coprocessor 310 is also a dual issue coprocessor. The two issue pipes within the coprocessor 310 are designated as *_0 and *_1, respectively. And, the coprocessor interfaces 312 of both the CPU 302 and the coprocessor 310 include interface signals corresponding to each of the two issue groups.

In clock cycle 1, two instructions A and B are dispatched by the coprocessor interface 312 within the CPU 302. Instruction A appears on signal lines CP_ir_0 corresponding to the first issue group (or pipe within coprocessor 310). Instruction B appears on signal lines CP_ir_1 corresponding to the second issue group (or pipe within coprocessor 310).

During clock cycle 2, the coprocessor interface 312 within the CPU 302 asserts strobe line CP1_as_0 to designate instruction A as an arithmetic instruction, to indicate to the coprocessor 310 that it is the target coprocessor for the instruction (i.e., CP1), to indicate that the pipe for which instruction A is intended is the “0” pipe, and to complete the transfer of instruction A to the coprocessor. Additionally, the coprocessor interface 312 within the CPU 302 asserts strobe line CP1_ts_1 to designate instruction B as a TO coprocessor data transfer instruction, to indicate to the coprocessor 310 that it is the target coprocessor for the instruction (i.e., CP1), to indicate that the pipe for which instruction B is intended is the “1” pipe, and to complete the transfer of instruction B to the coprocessor. While not shown, it should be appreciated that instruction validity or nullification also occurs during later clock cycles, similar to that illustrated in FIG. 4.

Also during clock cycle 2, instruction order signals are presented by the coprocessor interface 312 to the coprocessor 310 to indicate the programming order of instructions A and B. That is, even though instructions A and B are presented in parallel to the two instruction pipes “0” and “1” within the coprocessor 310, an indication must be presented to the coprocessor 310 to indicate their original order with respect to the program. Such order indication is necessary to insure that the coprocessor 310 resolves any dependencies between the instructions correctly.

Thus, each issue group carries an order signal to indicate the order of the instruction in that issue group with respect to the others. Two signal groups are provided between the CPU 302 and the coprocessor 310, corresponding to the two execution pipes within the coprocessor 310. The first signal group is CP_order_0, corresponding to the “0” pipe. The second signal group is CP_order_1, corresponding to the “1” pipe. Thus, during clock cycle 2, at the time instructions A and B are transferred by the strobe signals, the relative order of instructions A and B are designated by the two signal groups. More specifically, signal group CP_order_0 indicates to the “0” pipe in the coprocessor 310, that the instruction transferred to it, i.e., instruction A, is the first instruction, having a relative order of “0”. And, signal group CP_order_1 indicates to the “1” pipe in the coprocessor 310 that the instruction transferred to it, i.e., instruction B, is the second instruction, having a relative order of “1”.

During clock cycle 4, two more instructions, C and D are dispatched by the coprocessor interface 312 within the CPU 302 to the coprocessor 310. Instruction C is dispatched on signal group CP_ir_1 indicating that it is intended for the “1” pipe of coprocessor 310, and instruction D is dispatched on signal group CP_ir_0 indicating that it is intended for the “0” pipe of coprocessor 310. Note, if more than two coprocessors are coupled to the coprocessor interface 312 of the CPU 302, then the transfer of the dispatched instructions is designated by the strobe signals associated with each of the coprocessors (e.g., CP1 vs. CP2).

During clock cycle 5, transfer of instruction C to pipe “1” of the coprocessor 310 is completed by assertion of strobe line CP1_as_1. Assertion of this strobe line indicates that instruction C is an arithmetic instruction, and that it is directed to coprocessor CP1, in its “1” pipe. Also, transfer of instruction D to pipe “0” of the coprocessor 310 is completed by assertion of strobe line CP1_ts_0. Assertion of this strobe line indicates that instruction D is a TO coprocessor data transfer instruction, and that it is directed to coprocessor CP1, in its “0” pipe.

Also during clock cycle 5, the relative order of instructions C and D are provided to the coprocessor 310 via signal groups CP_order_0, and CP_order_1. Signal group CP_order_1 designates that instruction C is the first instruction, by providing a relative order quantity of “0” to the coprocessor 310. Signal group CP_order_0 designates that instruction D is the second instruction, by providing a relative order quantity of “1” to the coprocessor 310.

During clock cycle 6, instruction E is dispatched by the coprocessor interface 312 to the coprocessor 310.

During clock cycle 7, instruction E transfer is completed by assertion of strobe line CP1_as_0. This strobe line indicates that instruction E is designated for coprocessor CP1, that the instruction is an arithmetic instruction, and that it should be executed by the “0” pipe. Since instruction E was the only instruction dispatched during clock cycle 6, its relative dispatch order is provided by signal group CP_order_0 as “0”.

Also during clock cycle 7, instruction F is dispatched by the coprocessor interface 312 to the “1” pipe of the coprocessor 310.

During clock cycle 8, instruction F transfer is completed by assertion of strobe line CP1_ts_1. This strobe line indicates that instruction F is designated for coprocessor CP1, that the instruction is a TO coprocessor data transfer instruction, and that it should be executed by the “1” pipe. Since instruction F was the only instruction dispatched during clock cycle 7, its relative dispatch order is provided by signal group CP_order_1 as “1”.

Although not particularly illustrated, one skilled in the art should appreciate that by providing a 3-bit signal group for each available issue group, up to 8 instructions may be dispatched at the same time, while still tracking their relative order for instruction execution.

Instruction Transfer—Coprocessor Busy

Referring now to FIG. 6, a timing diagram 600 is shown which particularly illustrates the operation of various busy signals available on the coprocessor interface of the present invention. More specifically, each coprocessor that is coupled to a CPU may reach a point during execution of instructions such that it can't receive further instructions. Within the coprocessor interface of the present invention, busy signals are provided from the coprocessor to the CPU for each of the instruction types (a, t and f), for each available issue group.

During clock cycle 1, an instruction A is dispatched on signal group CP_ir_0. Also during cycle 1, a busy signal is asserted by coprocessor CPx on signal line CPx_tbusy_m. Assertion of this signal line indicates to the coprocessor interface 312 within the CPU 302 that coprocessor CPx cannot receive a TO coprocessor data instruction.

During clock cycle 2, instruction A transfer is completed by assertion of strobe line CPx_as_m. That is, although the coprocessor 310 asserted a busy signal, the busy portion of the coprocessor was related to TO coprocessor data instructions, not to arithmetic instructions. So, instruction A dispatch is attempted and transferred.

During clock cycle 3 instruction B is dispatched by the coprocessor interface 312. However, busy signals are being asserted by both the arithmetic and TO coprocessor data transfer portions of the coprocessor 310. So, the coprocessor interface 312 within the CPU 302 continues to attempt dispatch of instruction B until the busy signal corresponding to its instruction type is deasserted.

Also, during clock cycle 3, the arithmetic busy signal is deasserted. Since instruction B continues to be dispatched by the coprocessor interface 312, it appears that instruction B is not an arithmetic instruction.

During clock cycle 4, the TO coprocessor data transfer busy signal is deasserted. Since instruction B continues for 1 more clock cycle, it appears that instruction B is a TO coprocessor data transfer instruction.

During clock cycle 5, instruction B is dispatched.

During clock cycle 6, instruction B transfer is completed by assertion of strobe CPx_ts_m. Also during clock cycle 6, instruction C is dispatched by the coprocessor interface 312 within the CPU 302.

During clock cycle 7, instruction C transfer is completed by assertion of strobe signal CPx_fs_m, indicating that instruction C is a FROM coprocessor data transfer instruction.

During clock cycle 8, a busy signal is asserted on signal line CPx_fbusy_m to indicate that during cycle 8, the coprocessor 310 cannot receive another FROM data transfer instruction.

What should be appreciated from the above is that at any time during execution of instructions by a coprocessor, should that coprocessor's resources become consumed, within a particular group (a, t, or f), the coprocessor can prevent further instructions from being transferred that are specific to the particular group, without affecting transfer of instructions to other groups.

To/From Coprocessor Data Transfer

To/From data transfer refers to how data, corresponding to previously transferred instructions is provided from a CPU to a coprocessor, and vice versa. Table 2 below provides a brief summary of some of the data transfer signals that are provided within the COP interface of the present invention. Illustration of how these signals operate will be further described below with reference to FIGS. 7–9.

TABLE 2 Issue Signal Name Dir Group Description To Coprocessor Data (For all To Coprocessor Operations) CP_tds_m Out Comb, Coprocessor To Data Strobe. Asserted when To TF COP Op data is available on CP_tdata_m. CP_torder_m[2:0] Out Comb, Coprocessor To Order. Specifies which outstanding TF To COP Op the data is for. Valid only when CP_tds_m is asserted. CP_torder_m Order 3′b000 Oldest outstanding to COP Op data transfer 3′b001 2^(nd) oldest To COP OP data transfer 3′b010 3^(rd) oldest To COP OP data transfer 3′b011 4^(th) oldest To COP OP data transfer 3′b100 5^(th) oldest To COP OP data transfer 3′b101 6^(th) oldest To COP OP data transfer 3′b110 7^(th) oldest To COP OP data transfer 3′b111 8^(th) oldest To COP OP data transfer CP_tdata_m[63:0] Out Comb, To Coprocessor Data. Data to be transferred to the TF coprocessor. For single word transfers, data is valid on CP_tdata_m[31:0]. Valid when CP_tds_m is asserted. Note: In 32 bit data transfer size configuration, this bus is reduced to CP_tdata_m [31:0]. CP_tordlim_m[2:0] Sin Comb, To Coprocessor Data Out-of-Order Limit. This TF signal forces the integer processor core to limit how much it can reorder To COP data. The value on this signal corresponds to the maximum allowed value to be used on CP_torder_m[2:0]. From Coprocessor Data (For all From Coprocessor Operations) CP_fds_m In Comb, Coprocessor From Data Strobe. Asserted when TF From COP Op data is available on CP_fdata_m. CP_forder_m[2:0] In Comb, Coprocessor From Order. Specifies which out- TF standing From COP Op the data is for. Valid only when CP_fds_m is asserted. CP_forder_m Order 3′b000 Oldest outstanding From COP Op data transfer 3′b001 2^(nd) oldest From COP OP data transfer 3′b010 3^(rd) oldest From COP OP data transfer 3′b011 4^(th) oldest From COP OP data transfer 3′b100 5^(th) oldest From COP OP data transfer 3′b101 6^(th) oldest From COP OP data transfer 3′b110 7^(th) oldest From COP OP data transfer 3′b111 8^(th) oldest From COP OP data transfer CP_fdata_m[63:0] In Comb, From Coprocessor Data. Data to be transferred from TF coprocessor. For single-word transfers, data must be duplicated on both CP_fdata_m[63:32] and CP_fdata_m[31:0]. Valid when CP_fds_m is asserted. Note: In 32 bit data transfer size configuration, this bus is reduced to CP_fdata_m [31:0]. CP_fordlim_m[2:0] Sout Comb, From Coprocessor Data Out-of-Order Limit. This TF signal forces the coprocessor to limit how much it can reorder From COP Data. The value on this signal corresponds to the maximum allowed value to be used on CP_forder_m[2:0]

Referring now to FIG. 7, a block diagram 700 is shown illustrating particular data transfer signals 716 between a CPU 702 and a coprocessor 710 that were summarized above in Table 2. Each of the CPU 702 and the coprocessor 710 contain a COP interface 712 according to the present invention to allow data transfer as follows.

The coprocessor interface 712 within the CPU 702 transfers data to the coprocessor 710 after a TO coprocessor data transfer instruction has been dispatched. Only TO coprocessor data transfer instructions utilize this transfer. The coprocessor 710 must have a buffer available for this data after the To coprocessor data transfer instruction has been dispatched. If no buffers are available, the coprocessor 710 must prevent instruction dispatch by asserting CPx_tbusy_m.

The coprocessor interface 712 allows out-of-order data transfers. That is, data can be sent by the CPU 702 to the coprocessor 710 in a different order from the order in which instructions were dispatched. When data is sent to the coprocessor 710, the CP_torder_m[2:0] signal group is also sent. This signal group tells the coprocessor 710 if the data word is for the oldest outstanding To coprocessor data transfer instruction, or the 2^(nd) oldest, or the 3^(rd) oldest, and so on. The coprocessor interface 712 allows up to 7 transfers to be outstanding while returning data for the next transfer. The coprocessor 710 can limit the extent of this reordering to match what its hardware supports, by using the CP_tordlim_m[2:0] signal group.

To Coprocessor Data Transfer

Referring to FIG. 8, a timing diagram 800 is shown illustrating data transferred from the CPU 702 to the coprocessor 710 via the coprocessor interface 712 of the present invention.

During clock cycles 2, 4, and 6, To coprocessor data transfer instructions A, B, and C are dispatched by the coprocessor interface 712. These instruction transfers are completed with the assertion of the strobe signal CPx_ts_m in the following respective clock cycles.

During clock cycle 4, data corresponding to instruction A is presented on the signal group CP_tdata_m[63:0]. In the same clock cycle the data transfer is completed by assertion of strobe signal CP_tds_m. In addition, the relative order of the data being transmitted on the signal group CP_tdata_m is provided by the signal group CP_torder_m[2:0]. In this instance, since there are no outstanding To coprocessor data transfer instructions, the relative order of the data is “0”.

During clock cycle 7, data corresponding to outstanding To coprocessor data transfer instruction C is presented on signal group CP_tdata_m[63:0]. In addition, the data transfer is completed by assertion of strobe signal CP_tds_m. At this point, the data presented during clock cycle 7 is out-of-order with respect to its corresponding instruction C. That is, the data corresponding to instruction B has not yet been presented. This fact is communicated by the CPU 702 via signal group CP_torder_m when it presents a value of “1” on its signal lines. When the coprocessor interface 712 within the coprocessor 710 sees the “1” value on this signal group, it understands that the data being received is to be associated with the 2^(nd) oldest outstanding data transfer instruction, which in this case is instruction C.

During clock cycle 8, data associated with instruction B is presented on signal group CP_tdata_m. In addition, the data transfer is completed by assertion of strobe signal CP_tds_m. Finally, the relative order of the data being presented is provided via signal group CP_torder_m. In this instance, since the data associated with instruction A has already been provided, the data associated with instruction B is the oldest outstanding data. The coprocessor interface 712 within the CPU 702 therefore presents a value of “0” on signal group CP_torder_m.

Although not shown, what should be appreciated from the above is that out-of-order data transfers are independently tracked by the coprocessor interface 712 of the present invention, for each coprocessor 710 that is coupled to CPU 702, and for each issue group within each coprocessor 710.

From Coprocessor Data Transfer

The coprocessor interface 712 of the present invention transfers data from the coprocessor 710 to the CPU 702 after a From coprocessor data transfer instruction has been dispatched. Only From transfer instructions utilize this transfer. The CPU 702 must have available buffers to allow the transfer to occur in the cycle after dispatch. The coprocessor interface 712 allows out-of-order transfer of data similar to that described above with reference to To coprocessor data transfers.

Referring now to FIG. 9, a timing diagram 900 is shown illustrating the out-of-order data tracking mechanism associated with From coprocessor data transfer instructions, according to the coprocessor interface 712 of the present invention.

During clock cycles 2–4, and 6, From data transfer instructions A, B, C, and D are dispatched by the CPU 702 to the coprocessor 710. During clock cycles 3–5, and 7, instructions A–D are transferred to coprocessor 710.

During clock cycle 4, data associated with instruction A is presented by the coprocessor 710 to the CPU 702 via signal group CP_fdata_m. The data transfer is completed by assertion of strobe signal CPx_fds_m during the same clock cycle. At this point, the data being presented corresponds to data instruction A, which is the oldest outstanding From data transfer. Therefore, the relative order of the data is provided by the coprocessor interface 712 within the coprocessor 710, indicated on signal group CP_forder_m as “0”.

During clock cycle 5, data associated with instruction C is presented by the coprocessor 710 to the CPU 702. The data transfer is completed by assertion of strobe signal CP_fds_m during the same cycle. At this point, the data being presented corresponds to instruction C, which is the 2^(nd) oldest outstanding From data transfer, B data being the oldest. Therefore, the relative order of the data is provided by the coprocessor interface 712 within the coprocessor 710, indicated on signal group CP_forder_m as “1”.

During clock cycle 7, data associated with instruction D is presented by the coprocessor 710 to the CPU 702. The data transfer is completed by assertion of strobe signal CP_fds_m during the same cycle. At this point, the data being presented corresponds to instruction D, which is the 2^(nd) oldest outstanding From data transfer, B being the oldest. Therefore, the relative order of the data is provided by the coprocessor interface 712 within the coprocessor 710, indicated on signal group CP_forder_m as “1”.

During clock cycle 8, data associated with instruction B is presented by the coprocessor 710 to the CPU 702. The data transfer is completed by assertion of strobe signal CP_fds_m during the same cycle. At this point, the data being presented corresponds to instruction B, which is the oldest outstanding From data transfer. Therefore, the relative order of the data is provided by the coprocessor interface 712 within the coprocessor 710, indicated on signal group CP_forder_m as “0”.

Although not shown, what should be appreciated from the above is that out-of-order data transfers are independently tracked by the coprocessor interface 712 of the present invention, for each coprocessor 710 that is coupled to CPU 702, and for each issue group within each coprocessor 710.

Condition Code Checking

The coprocessor interface of the present invention provides signals for transferring the result of a condition code check from the coprocessor to the CPU. The instructions that typically utilize condition codes include branch instructions and conditional move instructions. When these instructions are decoded by the CPU, they are dispatched to both the CPU and to the coprocessor.

Table 3 below provides a brief summary of the condition code check signals that are provided within the COP interface of the present invention. Illustration of how these signals operate will be further described below with reference to FIGS. 10–11.

TABLE 3 Issue Signal Name Dir Group Description CP_cccs_m In Comb, Coprocessor Condition Code Check Arith Strobe. Asserted when condition code check results are available on CP_ccc_m. CP_ccc_m In Comb, Coprocessor Condition Code Check. Valid Arith when CP_cccs_m is asserted. When asserted, the instruction checking the condition code should proceed with its execution (i.e., branch or move data). When deasserted, the instruction should not execute its conditional operation (i.e., do not branch and do not move data)

For some instructions that are dispatched, a result is sent from the coprocessor to the CPU that indicates whether or not the CPU should execute that instruction. For branches, the coprocessor tells the CPU whether or not to branch. For example, a conditional branch executing in the CPU might require the evaluation of an operation whose operands occur within the coprocessor. In this instance, the evaluation of the operation is performed entirely within the coprocessor, and providing the final condition codes to the CPU, rather than transferring the operands from the coprocessor to the CPU, and evaluating the result in the CPU.

In the same way, if a conditional move instruction requires evaluation of an operation whose operands occur within the coprocessor, rather than transferring the operands from the coprocessor to the CPU, and then evaluating the result, evaluation of the operation is performed entirely within the coprocessor. That is, for conditional moves, the coprocessor tells the CPU whether or not to execute the move instruction.

Thus, within the coprocessor interface of the present invention, it is the coprocessor that interprets certain conditional branch and conditional move instructions, and decides whether or not the CPU should execute them. Since evaluation of CPU branch and conditional move instructions are performed in the coprocessor, user-defined Boolean conditional operations are possible for conditional branch, and conditional move instructions, i.e., they need not be fixed a priori by the integer core.

Referring now to FIG. 10, a block diagram 1000 is provided that illustrates the condition code signals 1016 transmitted by a coprocessor 1010 to a CPU 1002 via a coprocessor interface 1012. Operation of the condition code signals will now be described with reference to FIG. 11, to which attention is now directed.

FIG. 11 provides a timing diagram 1100 illustrating execution of three instructions, A, B, and C by the CPU 1002 and coprocessor 1010. Instruction A is dispatched by the CPU 1002 during clock cycle 2, and transfer is completed to the coprocessor 1010 by assertion of arithmetic strobe CPx_as_m during clock cycle 3. As mentioned above, instruction A also proceeds down the pipeline of the CPU 1002.

During clock cycle 4, the coprocessor interface 1012 of the coprocessor 1010 asserts the condition code strobe CP_cccs_m to indicate that the condition code resulting from evaluation of instruction A is available on condition code signal line CP_ccc_m. In this instance, CP_ccc_m is deasserted indicating to the CPU 1002 that it should not perform the operation specified by instruction A (either a conditional branch, or a conditional move operation).

Also during clock cycle 4, instruction B transfer is completed to the coprocessor 1010 by assertion of the arithmetic strobe CPx_as_m.

During clock cycle 5, the coprocessor interface 1012 of the coprocessor 1010 asserts the condition code strobe CP_cccs_m to indicate that the condition code resulting from evaluation of instruction B is available on condition code signal line CP_ccc_m. In this instance, CP_ccc_m is deasserted indicating to the CPU 1002 that it should not perform the operation specified by instruction B (either a conditional branch, or a conditional move operation). Also during clock cycle 5, instruction C is dispatched by the CPU 1002.

During clock cycle 6, instruction C transfer is completed to the coprocessor 1010 by assertion of the arithmetic strobe CPx_as_m.

During clock cycle 8, the coprocessor interface 1012 of the coprocessor 1010 asserts the condition code strobe CP_cccs_m to indicate that the condition code resulting from evaluation of instruction C is available on condition code signal line CP_ccc_m. In this instance, CP_ccc_m is asserted indicating to the CPU 1002 that it should perform the operation specified by instruction C (either a conditional branch, or a conditional move operation). Although not shown, it should be appreciated the “_m” suffix on the condition code signal lines indicate that condition codes exist for each issue group within a coprocessor's interface.

GPR Data

The coprocessor interface of the present invention provides signals for transferring the results of a check on a register within the CPU 1002 to the coprocessor 1010, for particular coprocessor instructions. These instructions include the MOVN.fmt and MOVZ.fmt instructions of the MIPS Instruction Set Architecture (ISA). Other instructions supported include the ALNV.PS and ALNV.fmt instructions. When these instructions are dispatched to the coprocessor 1010, they are also dispatched to the CPU 1002.

Table 4 below provides a brief summary of the GPR signals that are provided within the COP interface of the present invention.

TABLE 4 Signal Issue Name Dir Group Description CPx_(—) Out Comb, GPR Strobe. Asserted when additional general- gprs_m Arith purpose register information is available on CPx_gpr_m. CPx_(—) Out Comb, GPR Data. Supplies additional data from the gpr_m Arith CPU general purpose register file. CPx_gpr_(—) [3:0] m[2:0] is valid when CPx_gprs_m is asserted and only for ALNV.PS and ALNV.fmt instructions. CPx_pgr_m[3] is valid when CPx_gprs_m is asserted and only for MOVN.fmt and MOVZ.fmt instructions. CPx_gpr_m[2:0] RS (Valid only for ALNV.PS, ALNV.fmt) Binary encoded Lower 3bits of RS register contents CPx_gpr_m[3] RT Zero Check (Valid only for MOVN.fmt, MOVZ.fmt) 0 RT!=0 1 RT==0 Coprocessor Exception Found

The coprocessor interface of the present invention provides signals for transferring exception information from the coprocessor to the CPU. The exception found transfer is used to signal whether an instruction caused an exception in the coprocessor 1010, or not.

When a coprocessor instruction causes an exception, the coprocessor 1010 signals this to the CPU 1002 so that it can start execution from the exception vector.

Table 5 below provides a brief summary of the exception found signals that are provided within the COP interface of the present invention. Illustration of how these signals operate will be further described below with reference to FIGS. 10 and 12.

TABLE 5 Issue Signal Name Dir Group Description CP_excfs_m In Comb, Coprocessor Exception Found Strobe. Asserted Arith, when coprocessor exception signaling is available TF on CP_excf_m. CP_excf_m In Comb, Coprocessor Exception Found. When deasserted, the Arith, coprocessor is not causing an exception. When TF asserted, signifies that the coprocessor is causing an exception. The type of exception is encoded on the signal CP_exccode_m[4:0]. Valid when CP_excfs_m is asserted. CP_exccode_m[4:0] In Comb, Coprocessor Exception Code. Valid when Arith, CP_excfs_m is asserted and CP_excf_m is TF asserted. CP_(—) Exception exccode_m 5′b01010 Reserved Instruction Exception 5′b01111 Floating Point Exception 5′b10000 Available for implementation specific use 5′b10001 Available for implementation specific use 5′b10010 COP2 Exception Other values Reserved.

Referring now to FIG. 12, a timing diagram 1200 is shown illustrating the exception found signal generation provided by the coprocessor interface 1012 of the present invention.

Arithmetic instructions are dispatched by the CPU 1002 during clock cycles 2, 3 and 5. These instructions are transferred to the coprocessor 1010 by assertion of the arithmetic strobe signal CPx_as_m during clock cycles 3, 4 and 6, respectively.

For each of the instructions A, B, and C, exception found signals must be generated by the coprocessor 1010 to inform the CPU 1002 either that no exception occurred for the instruction, or that an exception occurred, along with an indication of the type of exception.

During clock cycle 4, the coprocessor interface 1012 within the coprocessor 1010 asserts exception found strobe CP_excfs_m to indicate that a coprocessor exception signal is available on CP_excf_m.

Also during clock cycle 4, the exception signal CP_excf_m is deasserted, thereby indicating that the oldest instruction for which an exception found signal has not been seen, in this case instruction A, did not generate an exception.

During clock cycle 5, the coprocessor interface 1012 within the coprocessor 1010 continues to assert exception found strobe CP_excfs_m to indicate that a coprocessor exception found signal is available on CP_excf_m.

Also during clock cycle 5, the exception signal CP_excf_m is deasserted, thereby indicating that the oldest instruction for which an exception found signal has not been seen, in this case instruction B, did not generate an exception.

During clock cycle 8, the coprocessor interface 1012 within the coprocessor 1010 asserts exception found strobe CP_excfs_m to indicate that a coprocessor exception found signal is available on CP_excf_m.

Also during clock cycle 8, the exception signal CP_excf_m is asserted, indicating that the oldest instruction for which an exception found signal has not been seen, in this case instruction C, did generate an exception.

Furthermore, the coprocessor interface 1012 within the coprocessor 1010 generates an exception code on the signal group CP_exccode_m[4:0], to indicate to the CPU 1002 what type of exception was generated by instruction C in the coprocessor.

Instruction Commit

The coprocessor interface of the present invention provides signals for notifying the coprocessor when instructions can or cannot commit state. All instructions dispatched utilize this transfer so that the coprocessor knows when it can write back results for the instruction.

Table 6 below provides a brief summary of the instruction commit signals that are provided within the COP interface of the present invention. Illustration of how these signals operate will be further described below with reference to FIGS. 10 and 13.

TABLE 6 Signal Issue Name Dir Group Description CP_(—) Out Comb, Coprocessor Commit Strobe. Asserted when commits_(—) Arith, commit signaling is available on m TF CP_commit_m. CP_(—) Out Comb, Commit Coprocessor Instruction. Valid when commit_(—) Arith, CP_commits_m is asserted. m[1:0] TF CP_(—) Type of Commit/Kill commit_(—) m[1:0] 2′b00 Instruction is not killed and can 2′b01 commit its results 2′b10 Instruction is killed. (not due to CP_excf_m) 2′b11 Instruction is killed (due to CP_excf_m)

Due to a variety of exceptional conditions, any instruction may need to be killed. The CPU 1002 contains logic that tells the coprocessor interface 1012 within the CPU 1002 when to commit or kill coprocessor instructions.

Occasionally, a coprocessor instruction will be killed because of a coprocessor signaled exception. For example, if a floating point instruction is killed because of a floating point exception, the coprocessor must update exception status bits in the coprocessor's status register. On the other hand, if that same instruction was killed because of a higher-priority exception, those status bits must not be updated. For this reason, as part of the commit when signaling a kill, the CPU 1002 tells the coprocessor if the instruction was killed due to a coprocessor signaled exception or not.

When a coprocessor arithmetic instruction is killed, all subsequent arithmetic instructions and To/From coprocessor data transfer instructions that have been dispatched are also killed. This is necessary because the killed instruction(s) may affect the operation of subsequent instructions. In the cycle in which an instruction is killed, other transfers may occur, but after that cycle, no further transfers occur for any of the killed instructions. A side-effect of this is that instructions subsequent to a killed instruction do not have a commit transfer of their own. In effect, they are immediately killed and thus their remaining transfers cannot be sent, including their own commit transfer. Previously nullified instructions do not have a commit transfer either, because once nullified, no further transfers occur for that instruction.

Referring to FIG. 13, a timing diagram 1300 is provided to particularly illustrate the instruction commit mechanism in the coprocessor interface of the present invention. Arithmetic instructions A, B, and C are dispatched by the CPU 1002 in clock cycles 2, 3, and 5, respectively. These instruction transfers are completed by the processor 1002 by assertion of the arithmetic strobe CPx_as_m during clock cycles 3, 4, and 6, respectively. Instruction commit information is then required for each of the instructions before the coprocessor is allowed to commit state.

During clock cycle 4, the commit strobe signal CP_commits_m is asserted by the coprocessor interface 1012 within the CPU 1002. This signal indicates that commit signaling is available on signal group CP_commit_m. Also during clock cycle 4, commit information is presented on signal group CP_commit_m by the coprocessor interface 1012 within the CPU 1002. The commit information presented on signal group CP_commit_m corresponds to the oldest instruction for which commit information has not yet been presented. In this case, the commit information corresponds to arithmetic instruction A.

In cycles 5 and 8, commit information associated with instructions B and C is presented on signal group CP_commit_m. Depending on the values presented on the signal group by the coprocessor interface 1012, the coprocessor 1010 will either commit or kill the associated instruction, according to the signal values shown in Table 6.

Miscellaneous Interface Signals

Table 7 below provides operational information relating to coprocessor interface signals that have not yet been discussed, but whose understanding is not enhanced by reference to a timing diagram.

TABLE 7 Issue Signal Name Dir Group Description CP_reset Out None Coprocessor Reset. Asserted when a hard or soft reset is performed by the CPU. At a minimum, this signal will be asserted for 1 cycle. CP_idle In None Coprocessor Idle. Asserted when the coprocessor logic is idle. Enables the integer processor core to go into sleep mode and shut down the internal integer processor core clock. Valid only if CP1_fppresent, CP1_mdmxpresent, or CPx_present is asserted. CP_tx32 SIn None Coprocessor 32bit Transfers. When asserted, the integer unit must cause an RI exception for 64bit TF instructions. Furthermore, when asserted, CP1_fppresent and CP1_mdmxpresent are ignored and internally deasserted. This forces CP_tx32 to only be used for COP2. This is a static input and must always be valid. CP1_fppresent Sin None Coprocessor FPU Present. Must be asserted when FPU hardware is connected to the coprocessor interface. CP1_mdmxpresent Sin None Coprocessor MDMX Present. Must be asserted when MDMX hardware is connected to the coprocessor interface. CPx_present Sin None Coprocessor Present. Must be asserted when coprocessor hardware is connected to the coprocessor interface. CP_irenable_m Out Comb, Enable Instruction Registering. When deasserted, no Arith, instruction strobes will be asserted in the following cycle. TF When asserted, there may be an instruction strobe asserted in the following cycle. Instruction strobes include CPx_as_m, CPx_ts_m, CPx_fs_m. CP_adisable_m Sin Comb, Inhibit Arithmetic Dispatch. When asserted, prevents the Arith CPU from dispatching an arithmetic instruction using this issue group. CP_tfdisable_m Sin Comb, Inhibit To/From Dispatch. When asserted, prevents the TF CPU from dispatching a To/From instruction using this issue group. CP_inst32_m Out Comb, MIPS32 Compatibility Mode-Instructions. When Arith, asserted, the dispatched instruction is restricted to the TF MIPS32 subset of instructions. Please refer to the MIPS64 architecture specification available from MIPS Technologies, Inc. for a complete description of MIPS32 compatibility mode. Valid the cycle before CPx_as_m, CPx_fs_m, or CPx_ts_m is asserted. CP_fr32_m Out Comb, MIPS32 Compatibility Mode-Registers. When Arith, asserted, the dispatched instruction uses the MIPS32 TF compatible register file. Valid the cycle before CPx_as_m, CPx_ts_m, or CPx_fs_m is asserted. CP_endian_m Out Comb, Big Endian Byte Ordering. When asserted, the processor Arith, is using big endian byte ordering for the dispatched TF instruction. When deasserted, the processor is using little- endian byte ordering. Valid the cycle before CPx_as_m, CPx_ts_m, or CPx_fs_m is asserted. Various Issue Group Configurations

A CPU that supports only single-issue integer instructions may implement a single combined issue group that can issue any type of instruction as follows:

The issue group will be 0 (m=0).

CP_adisable_0 and CP_tfdisable_0 may not be implemented. Since this is the only issue group, these instructions can never be disabled.

CP_order_0[2:0] may not be implemented. Since there is only one issue group, dispatch order is not needed.

An integer core with this configuration can be used with a coprocessor with more issue groups. In this case, the Combined issue group of the coprocessor is connected to the combined issue group of the integer processor core and the other issue groups of the coprocessor are tied inactive.

An integer processor core that supports limited dual issue supports dual-issue of instructions only when one is an arithmetic coprocessor instruction and the other is a To/From coprocessor instruction. With this option, two issue groups will be implemented—one combined (issue group 0) and one arithmetic (issue group 1).

An integer core that supports full dual issue supports all the cases of limited dual issue, plus it can issue two arithmetic instructions or two To/From instructions. With this option, two combined issue groups will be implemented.

A single-issue coprocessor can be used with a dual-issue integer processor core by simply connecting the combined issue groups together and asserting CP_adisable_1 and CP_tfdisable_1 for the 2^(nd) combined issue group of the integer processor core.

A limited dual-issue coprocessor can be used with a dual-issue integer processor core by connecting the coprocessor combined issue group to one of the integer processor core's combined issue groups and asserting CP_adisable_m for that issue group. Then, connect the coprocessor's arithmetic issue group to the remaining combined issue group of the integer processor core and assert CP_tfdisable_m for that issue group.

Although the present invention and its objects, features, and advantages have been described in detail, other embodiments are encompassed by the invention. In addition to implementations of the invention using hardware, the invention can be embodied in software disposed, for example, in a computer usable (e.g., readable) medium configured to store the software (i.e., a computer readable program code). The program code causes the enablement of the functions or fabrication, or both, of the invention disclosed herein. For example, this can be accomplished through the use of general programming languages (e.g., C, C++, etc.), hardware description languages (HDL) including Verilog HDL, VHDL, AHDL (Altera Hardware Description Language) and so on, or other programming and/or circuit (i.e., schematic) capture tools available in the art. The program code can be disposed in any known computer usable medium including semiconductor memory, magnetic disk, optical disc (e.g., CD-ROM, DVD-ROM, etc.) and as a computer data signal embodied in a computer usable (e.g., readable) transmission medium (e.g., carrier wave or any other medium including digital, optical or analog-based medium). As such, the code can be transmitted over communication networks including the Internet and intranets. It is understood that the functions accomplished and/or structure provided by the invention as described above can be represented in a core that is embodied in program code and may be transformed to hardware as part of the production of integrated circuits. Also, the invention may be embodied as a combination of hardware and software.

Furthermore, the particular illustrations used to describe the coprocessor interface of the present invention included a single CPU and a single coprocessor. However, as mentioned above, the coprocessor interface has been described to encompass connections between multiple CPU's and multiple coprocessors, any of which may have single issue pipelines, or multiple issue pipelines.

Also, although the coprocessor interface has been described with particular reference to MIPS32 and MIPS64 Instruction Set Architecture, one skilled in the art will appreciate that the scalability, and configurability of the interface is not limited to such architecture, nor is it limited to the data width of the instructions, and/or the data that is being transferred.

Finally, those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiments as a basis for designing or modifying other structures for carrying out the same purposes of the present invention without departing from the spirit and scope of the invention as defined by the appended claims. 

1. An interface for transferring data between a central processing unit (CPU) and a plurality of coprocessors, the interface comprising: an instruction bus, configured to transfer instructions to the plurality of coprocessors in an instruction transfer order, wherein particular instructions designate and direct one of the plurality of coprocessors to transfer the data to/from the CPU; and a data bus, coupled to said instruction bus, configured to transfer the data, wherein data order signals within said data bus specify a data transfer order that differs from said instruction transfer order, and wherein said data order signals specify transfer of a data element, said data element corresponding to a specific outstanding instruction, wherein said data order is relative to outstanding instructions, said outstanding instructions being those of said particular instructions transferred to said one of the plurality of coprocessors that have not completed a data transfer; wherein the interface keeps track of said data order, and wherein said data order signals indicate said data order, and wherein said data order signals are provided with said data element as said data element is transferred.
 2. The interface as recited in claim 1 wherein the plurality of coprocessors comprises: a first plurality of floating-point coprocessors; or a first plurality of graphics (3-D) coprocessors; or a second plurality of floating-point coprocessors and a second plurality of graphics (3-D) coprocessors.
 3. The interface as recited in claim 1, wherein said particular instructions comprise TO instructions, said TO instructions directing that the transfer of the data will be from the CPU to said one of the plurality of coprocessors.
 4. The interface as recited in claim 3, wherein said particular instructions comprise FROM instructions, said FROM instructions directing that the transfer of the data will be to the CPU from said one of the plurality of coprocessors.
 5. The interface as recited in claim 4, wherein said data bus comprises: data TO signals, for transferring data from the CPU to one of the plurality of coprocessors; and data FROM signals, for transferring data to the CPU from said one of the plurality of coprocessors.
 6. The interface as recited in claim 5, wherein said data order signals comprise: TO order signals, for specifying said data transfer order with respect to transfers via said data TO signals; and FROM order signals, for specifying said data transfer order with respect to transfers via said data FROM signals.
 7. The interface as recited in claim 6, wherein said TO order signals prescribe a particular outstanding TO instruction that is relative in order to all outstanding TO instructions.
 8. The interface as recited in claim 6, wherein said FROM order signals prescribe a particular outstanding FROM instruction that is relative in order to all outstanding FROM instructions.
 9. The interface as recited in claim 1, wherein said data bus transfers the data over parallel signal lines to said one of the plurality of coprocessors, wherein said one of the plurality of coprocessors has multiple issue pipelines providing for parallel instruction execution.
 10. A computer program product for use with a computing device, the computer program product comprising: a computer usable medium, having computer readable program code embodied in said medium, wherein said computer readable program code, when used with the computing device, causes enablement of functions corresponding to a coprocessor interface that transfers data between CPU and a plurality of coprocessors, said computer readable program code comprising: first program code, for causing enablement of functions corresponding to an instruction bus, said instruction bus configured to transfer instructions to said plurality of coprocessors in an instruction transfer order, wherein particular instructions designate and direct one of said plurality of coprocessors to transfer said data to/from said CPU; and second program code, for causing enablement of functions corresponding to a data bus, said data bus configured to transfer said data, wherein data order signals within said data bus specify a data transfer order that is different from said instruction transfer order, and wherein said data order signals specify transfer of a data element, said data element corresponding to a specific outstanding instruction, wherein said data order is relative to outstanding instructions, said outstanding instructions being those of said particular instructions transferred to said one of said plurality of coprocessors that have not completed a data transfer; wherein the coprocessor interface keeps track of said data order, and wherein said data order signals indicate said data order, and wherein said data order signals are provided with said data element as said data element is transferred.
 11. The computer program product as recited in claim 10, wherein said particular instructions comprise: TO instructions, said TO instructions directing that the transfer of said data will be from said CPU to said one of said plurality of coprocessors; and FROM instructions, said FROM instructions directing that the transfer of said data will be to said CPU from said one of said plurality of coprocessors.
 12. The computer program product as recited in claim 11, wherein said data order signals comprise: TO order signals, for specifying said data transfer order for a particular outstanding TO instruction that is relative in order to all outstanding TO instructions; and FROM order signals, for specifying said data transfer order for a particular outstanding FROM instruction that is relative in order to all outstanding FROM instructions.
 13. The computer program product as recited in claim 10, wherein said data bus is configured to transfer said data over parallel signal lines to particular coprocessors that have multiple issue pipelines providing for parallel instruction execution and corresponding data transfers.
 14. A method for transferring data between a CPU and a plurality of coprocessors, the method comprising: transmitting instructions to one of the plurality of coprocessors, each of the instructions designating the one of the plurality of coprocessors, and each of the instructions directing a data transfer between the CPU and the one of the plurality of coprocessors, wherein said transmitting is provided in a specific instruction order; and transferring the data in an order different from the specific instruction order, said transferring comprising: specifying transfer of a data element corresponding to a specific outstanding instruction, wherein the order of said transferring is relative to outstanding instructions, the outstanding instructions being those of the instructions transmitted to the one of the plurality of coprocessors that have not completed a data transfer; and providing the data order signals with the data element as the data element is transferred.
 15. The method as recited in claim 14, said transmitting comprises: issuing a plurality of the instructions over parallel signal lines to the one of the plurality of coprocessors; and designating an execution order for the plurality of the instructions. 